The present invention is directed to a method for removing mercury in a flue gas desulfurization process.
Wet flue gas desulfurization is a process for removing sulfur compounds from the waste gases produced, for example, in power stations, waste incineration plants, or by large engines. The sulphur compounds arise due to the incineration of fuels containing sulfur, which are mainly fossil fuels. The plants for flue gas desulfurization are abbreviated as FGD (flue gas desulfurization plant). A flue gas desulfurization plant can also be used for the recovery of gypsum (FGD gypsum). This type of gypsum recovery has been in the state of the art for a long time.
In this known process, the gypsum suspension that collects in the scrubber contains the mercury. This suspension is thickened using at least one gypsum cyclone (hydrocyclone) and the thickened gypsum suspension is discharged with the underflow from the hydrocyclone. The overflow from the hydrocyclone is fed to at least one waste water cyclone to recover the solids, and the overflow from the waste water cyclone is then treated or cleaned subsequently in a waste water treatment plant (WWTP). The thickened suspension in the underflow from the hydrocyclone is further dewatered to the final dry content by band filters or a centrifuge. The gypsum quality normally has to satisfy requirements as to degree of purity.
In addition to desulfurization in the wet scrubber, mercury and halogen compounds contained in the flue gas are also removed and carried out of the scrubber with the gypsum suspension. When mercury is removed from combustion gases, especially when generating electricity from coal, plants are confronted with the problem of the mercury already removed from the flue gas, being re-emitted from the wet scrubber.
Oxidized mercury species (in the form of HgX2, X═Cl, Br, I) are soluble in water and can thus be removed effectively from the flue gas together with the other acid components in the flue gas. Depending on the washer chemistry, the mercury integrated into the wash water is dissolved (but not generally dissociated); as an alternative, complexing by means of halogen salts or an adsorptive bond, especially to fines containing carbon, is possible. Dissolved (oxidized) mercury can re-emit due to its own steam pressure; as an alternative, reductive destruction of the compound is possible, releasing elementary (water-insoluble) mercury, which then re-emits entirely as a result.
This problem can be dealt with by transforming the dissolved mercury species into an adsorbed form. This could be achieved with adsorbents based on active coal, for example, or by precipitating the relevant contaminants in the washer loop (e.g., with sulfide precipitants). The adsorptively bound mercury is found preferentially on a fine-particle fraction, while coarse-grained solids remain more or less mercury-free.
The gypsum recovery systems commonly used in the wet flue gas desulfurization process (see FIG. 1) mainly comprise a gypsum cyclone 11 followed by a belt filter (not shown in FIG. 1) and a waste water cyclone 12 for recovery of solids before the waste water treatment plant 13. The hydrocyclones used here are distinguished by the fact that they concentrate particles that sink more quickly in the underflow. All particles are concentrated here regardless of their size (except colloidal systems) if they have a higher density than the fluid in the underflow. Nearly all large particles are removed, and very small ones only accumulate to a small extent.
The result is increased mercury input to the cyclone underflow and thus to the FGD gypsum product because the mercury is attached to the fines fraction (which also has a higher concentration). Depending on the operating mode and volume of waste water/type of cyclone, up to 90% of the mercury load can end up in the gypsum. The volume-related mercury content in the overflow is reduced slightly via the cyclone stage. That means that the concentration discharged after a two-stage cyclone process settles under the particles in the washer.
This can be improved by using the wash water cyclone technology described in PCT/EP2013/000379, now U.S. Pat. No. 9,139,449. With this technology, additional water is added in order to reduce the concentration of fines in the underflow. Systems of this kind reduce the mercury load in the cyclone underflow by ˜20-30%. However, the gypsum continues to be the main point of accumulation in the process. The cyclone overflow has slightly higher mercury content than when using a conventional cyclone.
This problem becomes more relevant against the backdrop of the discussion on emission limits for mercury because a stringent tightening of limits is expected. In view of the prevailing plant arrangements depicted in FIG. 1 (boiler-SCR-ESP-WFGD), the most economical separation process can be expected in the washer.
Two-stage gypsum dewatering (gypsum washing) with intermediate dilution using process water would be a feasible variant. In this way, highly pure and almost mercury-free gypsum would be conceivable, although the Hg inventory in the washer would then continue to increase until the load incorporated (by means of flue gas) reaches the same level as the load discharged in the waste water flow. This status requires very high mercury concentrations in the washer, which in turn tend to cause greater re-emission into the clean gas.
The problem is that coal-fired power stations usually have relatively small amounts of waste water. In relation to the electricity required for preliminary dewatering of gypsum, the waste water quantity is in a range <10%, often in a range between 5 and 7%. In order to ensure high mercury load in the waste water treatment plant, the concentration must be correspondingly high, which leads in turn to increased re-emissions in the washer.
Accordingly, if the Hg transfer to the gypsum is to be reduced, the waste water quantity must be increased. However, this means that more equipment is required in the waste water cleaning plant. As an alternative, the Hg transfer to the FGD gypsum can be reduced with a modified form of gypsum pre-dewatering (using a wash cyclone). However, this means a significant increase in Hg in the washer with a higher tendency towards re-emission if no additional measures are implemented.